In the related art, there is known a post-processing apparatus, which executes post-processing such as stapling processing on sheets stacked onto a processing tray. The sheet post-processing apparatus includes a lateral alignment member that aligns sheets misaligned in the width direction (lateral alignment processing) and a longitudinal alignment member that aligns the sheets misaligned in the direction that is orthogonal to the sheet width direction of the sheets (longitudinal alignment processing) in order to align misaligned sheets stacked onto the processing tray before the post-processing is performed. In particular, the sheet post-processing apparatus in the related art uses the longitudinal alignment member that rotates about a rotating shaft extending to the sheet width direction in order to clear the misalignment of sheets in the direction that is orthogonal to the width direction of the sheets.
However, in the sheet post-processing apparatus in the related art, there is a problem that once any external force is applied to the sheets after longitudinal alignment processing or lateral alignment processing are executed, the aligned sheets are misaligned.